Acrylamide is widely used as the starting monomer for polyacrylamide which has a wide variety of utility such as coagulant and paper-strength agent. It is usually available in the form of aqueous solutions the concentrations of which range from 30 to 50 wt. %. When shipping acrylamide to a distant destination or storing same in a cold district or depending on the application field of acrylamide, it is required to provide acrylamide in its crystalline form.
Crystalline acrylamide is usually produced by concentrating and cooling an aqueous solution of acrylamide to crystallize it out, followed by its filtration and drying. There is a standing demand for the development of not only a proficient concentration technique but also an efficient cooling technique.
If one tries to cool an aqueous solution of acrylamide to crystallize out the acrylamide by using a heat exchanger equipped with conventional cooling means such as coil, jacket or the like, scale of acrylamide crystals is caused to deposit on the cooling surfaces and the quantity of transferable heat is reduced extremely, thereby inhibiting efficient cooling and crystallization.
In order to overcome the above-mentioned problem, the vacuum crystallization process has heretofore been employed principally so that an aqueous solution of acrylamide is depressurized to have its water evaporate, thereby taking its latent heat of vaporization from the solution and hence cooling the thus-concentrated solution and causing the acrylamide to crystallize out (see, for example, Japanese Patent Laid-Open No. 115410/1976).
In an investigation conducted by the present inventors, crystallization under the usual conditions proposed in Japanese Patent Laid-Open No. 115410/1976, namely, at a temperature of 5.degree. C., a pressure of 5-6 torr and an oxygen partial pressure as low as 0.25 torr encountered such serious problems that the polymerization of acrylamide was not successfully prevented and the resultant polymer plugged the crystallizer and/or was allowed to mix in the intended final product, i.e., acrylamide crystals. The crystallizer is a vacuum apparatus and hence requires a substantial initial cost. It has also been found that the operation is very cumbersome.
With the foregoing in view, it came to the positive attention of the present inventors that these problems could be solved if a cooling and crystallization process making use of a heat exchanger may be used in place of the vacuum crystallization process.
For example, according to "Kagaku Kogaku Binran (Handbook of Chemical Engineering)", revised 3rd edition, page 437 (1968) published by Maruzen Co., Ltd., Tokyo, Japan, it is indicated to be effective for the prevention of scale deposition in apparatus and circulation systems to improve the surface finishing of each cooling surface to reduce its surface roughness, to apply a lining to each cooling surface with an organic material or the like.
According to an investigation conducted by the present inventors, no additional effects were however observed for the prevention of scale deposition even when the surface roughness R.sub.max of each sample surface, as measured by JIS B-0601-1982 or ISO R 468-1966 for instance, was improved to 0.9.mu. or so by the so-called buffing as described, for example, in "Shimpan Hyomen Shori Handbook (Surface Treatment Handbook, New Edition)", page 110-115 (1969) published by The Sangyo Tosho Kabushiki Kaisha, Tokyo, Japan, namely, by finishing the surface successively, for example, with #100, #150, #300 and #400 abrasives while using paper buff and then loose buff, and finally mirror-finishing the surface with green rouge. The term "R.sub.max " as used herein means the distance between the top and bottom of a rough area in the surface of a material as expressed in terms of micrometer (.mu.m) as shown in the accompanying drawings, which will be described later in this specification. It has hence been found that the deposition of scale cannot always be prevented even if the surface roughness is reduced.
Regarding the prevention of polymer deposition on the inner wall of a polymerization tank for vinyl chloride or its analogous monomer, Japanese Patent Laid-Open No. 29389/1979 discloses to polish the inner wall mechanically with buff of #400 or more precise and then to mirror finish the thus-polished inner wall with an abrasive compound such as finishing green rouge; or to polish the inner wall mechanically with buff of #300 or greater and then to subject the thus-polished inner wall to electrolytic polishing in order to mirror finish same. Namely, the above process intends to prevent the deposition of the resultant polymer by reducing the surface roughness of the inner wall through its buffing or the like. Electrolytic polishing is employed as one of means for achieving that goal.
Turning finally to the application of a lining or the like with an organic material, no effects were observed at all when phenol resins were coated. Even when a fluoroplastic lining was applied, it was impossible to prevent the deposition of scale sufficiently. Moreover, such a lining has a small heat transfer coefficient and a very large cooling area is therefore required. Use of such a lining raises certain problems in manufacturing a heat exchanger.